This invention relates generally to marking techniques for semiconductor wafers and devices. More specifically, the present invention relates to methods and apparatus using laser and other optical-energy reactive materials for marking the surface of a bare semiconductor die.
An individual integrated circuit semiconductor die or chip is usually formed from a larger structure known as a semiconductor wafer, which is typically comprised primarily of silicon, although other materials such as gallium arsenide and indium phosphide are also sometimes used. Each semiconductor wafer has a plurality of integrated circuits arranged in rows and columns with the periphery of each integrated circuit being substantially rectangular. In response to the ever-increasing demand for smaller, higher performance semiconductor dice, wafers are typically thinned (i.e., have their cross sections reduced) by a mechanical and/or chemical grinding process. After thinning, the wafer is sawn or xe2x80x9cdicedxe2x80x9d into rectangularly shaped discrete integrated circuits along two mutually perpendicular sets of parallel lines (streets) lying between each of the rows and columns thereof on the wafer. Hence, the separated or singulated integrated circuits are commonly referred to as semiconductor die or semiconductor dice. While semiconductor dice may carry information of the active surface thereof regarding the manufacturer, specifications, etc., such information cannot be easily read without the use of optical devices. Subsequent to the wafer-dicing process, individual semiconductor dice are commonly subjected to a marking process wherein various easily read information is placed on the backside or inactive side of the semiconductor die for purposes of corporate identity, product differentiation and counterfeit protection.
Recently, lasers have supplanted the ink stamping process as the quickest and most efficient way to mark finished bare semiconductor dice or packaged semiconductor dice. Thus, lasers are currently used to mark semiconductor dice with a manufacturer""s logo, as well as alphanumeric marks and bar codes specifying the company""s name, a part or serial number, or other information such as lot or die location. In particular, lasers have become especially useful in marking high production items such as bare or packaged semiconductor dice. The high speed and precision of laser marking makes their use highly desirable for high throughput automated processes.
Conventional laser marking techniques utilize a very high intensity beam of light to alter the surface of a semiconductor die directly by melting, burning, or abating the device surface directly, or by discoloration or decoloration of a laser reactive coating applied to a surface of the bare semiconductor die or packaged semiconductor die. The beam of light may be scanned over the surface of the bare semiconductor die or packaged semiconductor die in the requisite pattern, or can be directed through a mask which projects the desired inscriptions onto the desired surface of the bare semiconductor die or packaged semiconductor die. The surface or coating of the bare or packaged semiconductor die thus modified, the laser marking creates a reflectivity different from the rest of the surface of the bare or packaged semiconductor die.
Numerous methods for laser marking are known in the art. One method of laser marking involves applications where a laser beam is directed to contact the surface of a semiconductor device directly, as is illustrated in U.S. Pat. No. 5,357,077 to Tsuruta, U.S. Pat. No. 5,329,090 to Woelki et al., U.S. Pat. No. 4,945,204 to Nakamura et al., U.S. Pat. No. 4,638,144 to Latta, Jr., U.S. Pat No. 4,585,931 to Duncan et al., and U.S. Pat. No. 4,375,025 to Carlson. In these direct marking applications, the roughness of the laser-marked surface is different from that of the unmarked surface. Thus, the contrast generated by this type of laser marking is the result of several factors, including surface depressions and asymmetry in surface lines. The inscriptions created by burning the surface of the semiconductor die can therefore be read by holding the device at an angle to a light source. An additional factor that may affect the contrast is the remnants of any burnt compounds generated by the laser marking which have a different reflectivity from the original material.
Another method of laser marking makes use of various surface coatings, e.g., carbon black and zinc borate, of a different color than the underlying device material. When the laser heats the coating to the point of vaporization, a readable mark is created by virtue of the contrast in the two layers. An example of this type of marking method was described in U.S. Pat. No. 4,707,722 to Folk et al. The methods disclosed by Folk involve the deposition of an ablative coating made of electroless nickel layer, in a form highly absorptive of radiant energy, on a surface of a metal package. The ablative coating is then vaporized by a laser, allowing the shiny metal of the package to show through in the form of a mark.
A further method used in the marking of a chip uses materials known in the art to be capable of changing color when contacted by a laser beam. For example, U. S. Pat. No. 5,985,377 to Corbett, assigned to the assignee of the present invention, describes a laser reactive material, such as a material containing a B-stage epoxy with an added pigment of a desired color, that reacts with heat to form a new compound on the surface of the chip and subsequently cures to a desired color. Corbett additionally discloses use of an ink-bearing material, such as a ribbon, which transfers ink to the surface of a chip when exposed to a laser. U.S. Pat. No. 4,861,620 to Azuma discloses a laser-reactive coating formed of various pigments, incorporating mercury and other heavy metals, which will thermally decompose, and hence change colors, when heated to a predetermined temperature by a laser beam. The result is a mark having a different color from the background color of the chip package.
U.S. Pat. No. 4,753,863 to Spanjer describes a laser-markable molding compound incorporating titanium oxide and/or chromium oxide as a coloring material, polyamide, epoxy, or silicone as a plastic resin, and a filler made of silicon oxide or aluminum oxide. When exposed to a laser, the originally grey molding composition turns a bright gold color. U.S. Pat. No. 5,928,842 to Shinmoto et al. discloses a silicon and polyolefin resin-based marking composition which a laser will turn from dark brown to black.
Each of these marking methods, however, is subject to a number of drawbacks and limitations. In methods involving the laser marking of a bare die, the ideal result is that the burned portion of the surface of the semiconductor die becomes sufficiently roughened to become visibly distinguishable from the semiconductor die""s intact smooth surface. However, the laser mark is not always easily recognizable due to insufficient contrast between the roughened and smooth surfaces. This is particularly the case with semiconductor dice that have been subjected to backgrinding as part of a wafer thinning process.
As a result of wafer thinning, the grinding wheel used to abrade silicon from the backside of a wafer having a plurality of locations of semiconductor dice formed thereon tends to create swirling patterns on the backside surface of the wafer and portions of swirling patterns on the backside surface of the semiconductor dice. These swirling patterns or portions thereof may be sufficiently rough to interfere with an ablative laser processxe2x80x94making it much more difficult to burn a distinguishing mark on the surface of the semiconductor die. As a further result of the operation of the grinding wheel, the pattern left by the grinding process varies for semiconductor dice taken from one side of the wafer as opposed to the other, thus adding to the difficulty of reading the mark. An additional problem with bare die laser marking is that the high intensity of the laser beam may cause thermal degradation of the bare or packaged semiconductor die, or even damage the internal circuitry of the semiconductor die directly.
Secondly, use of laser reactive coatings may not be advantageous for use in a high-throughput process since many of them take hours to cure. Moreover, many laser coatings will lose the desired degree of contrast when exposed to the elevated temperatures prevalent in semiconductor die burn-in tests. Further considerations may weigh against coatings that incorporate unsafe heavy metals, as well as coatings that add alpha particle or mobile ion sources known to cause degradation of semiconductor dice. Finally, many coatings are difficult and expensive to apply as they require the use of special apparatus and/or costly materials.
Accordingly, there exists a need for an inexpensive, quick, high-resolution, and high-quality mark that is compatible with existing semiconductor fabrication and testing processes. Two of several phases of the fabrication process that lend themselves to the introduction of a complementary technique for preparing semiconductor dice for laser marking are the backgrinding and dicing processes.
During conventional back-surface grinding treatments, a semiconductor wafer is thinned to a desired thickness by the mechanical action of a grinding wheel. In processing the semiconductor wafer, the circuit pattern-formed surface (the xe2x80x9cactive surfacexe2x80x9d) of the e wafer is prevented from being stained or injured with grinding t rashes, etc., by a protective member or submount previously adhered to the circuit pattern-formed surface of the wafer via an automatic adhering apparatus. After applying the back surface grinding treatment, the protective member may remain or may be peeled off or dislodged, and the semiconductor wafer is sent to a subsequent dicing process. To support and transport the wafer for dicing, a carrier tape or film is typically applied to the back surface of the wafer. Following dicing, the semiconductor dice are marked with identifying information, and either stored, transported, or mounted on carrier substrates such as leadframes or circuit boards which will be populated with an individual semiconductor die or semiconductor dice. The carrier tape or film applied prior t o dicing is typically removed during the pick-and-place process of attaching singulated dice to the desired carrier substrate.
The present invention provides a method and apparatus for marking a semiconductor wafer or semiconductor die. The method and apparatus have particular application to wafers or semiconductor dice which have been subjected to a thinning process, including backgrinding in particular. The present method comprises reducing the cross-section of a wafer or semiconductor die, applying a tape having optical energy-markable properties over a surface or edge of the wafer or semiconductor die, and exposing the tape to an optical energy source to create an identifiable mark. In one embodiment, a markable tape of the present invention is applied to a surface which has been roughened by exposure to an abrasive thinning process. The application of the tape creates a homogeneous surface suitable for the formation of an optical energy-induced mark, such as that formed by a laser, the mark rendered readable by virtue of the contrast provided by the tape. In a related embodiment, an adhesive affixed to a tape provides the markable properties of the tape. All, part, or only a residue of the tape may remain on the wafer or semiconductor die after the marking process. In this regard, the tape or a markable adhesive affixed thereon may be advantageously formed of thermally dissipating and/or anti-static types of materials. The tape additionally may have a coefficient of thermal expansion similar to, or the same as, the materials in the wafer or the device to which it is applied. Carrier tapes are also disclosed for complementary use in the present method wherein the tapes may be formed to have translucent properties, or to provide additional marking qualities.
In another embodiment, the markable tape is used in a method of manufacturing an integrated circuit semiconductor die. This method basically entails providing a semiconductor wafer, reducing the cross-section of the semiconductor wafer (for example, by backgrinding), applying the markable tape, dicing the wafer, and subjecting the diced wafer or individual die to an optical energy source to render a mark.
The invention further provides a method for identifying a known good die (KGD). In this method, various identifying test data are compiled and incorporated into an optical energy-generated mark which is formed after the application of a markable tape.
The invention also includes a laser-markable tape apparatus for use in marking bare semiconductor dice. The apparatus comprises a tape which makes use of a multi-level adhesive that includes an outermost layer formed of a mixture of electromagnetic radiation-curing components and adhesive. After application to a bare semiconductor die and exposure to an electromagnetic radiation source, the mixture layer cures and bonds to the die surface, rendering a homogeneous surface suitable for laser marking.